1. Field of the Invention
The present invention relates to an electronic device having a ferroelectric layer and a method of manufacturing the same, more particularly to an electronic device having a ferroelectric layer oriented crystallographically and a method of manufacturing the same.
2. Description of the Related Art
A semiconductor memory, in which one memory cell is constituted of one transistor and one capacitor, has been widely known. A capacitor of a dynamic random access memory (DRAM) has a capacitor dielectric layer formed of a paraelectric material. Electric charges stored in the capacitor gradually decrease therefrom due to their leak even when the transistor is turned off. Accordingly, when a voltage applied to the memory cell is removed, information stored therein decreases and disappears before long.
A memory capable of retaining information stored therein even after power is cut off is called a non-volatile memory. As a kind of the non-volatile memory, a one-transistor/one-capacitor type memory, a capacitor dielectric layer of which is formed of a ferroelectric material, has been known, which is called a ferroelectric random access memory (FeRAM).
The FeRAM utilizes residual polarization of the ferroelectric material as information stored therein. The FeRAM controls a polarity of a voltage applied between a pair of electrodes of the ferroelectric capacitor, thus controlling the direction of the residual polarization. Assuming that one polarization direction be “1” and the other be “0”, binary information can be stored. Since the residual polarization remains in the ferroelectric capacitor even after the applied voltage is removed therefrom, the non-volatile memory can be realized. In the non-volatile memory, information can be rewritten by a sufficient number of times, that is, 1010 to 1012 times. The non-volatile memory also has a rewriting speed of an order of several ten nanoseconds and offers a high-speed operability.
As ferroelectric materials, lead-based oxide ferroelectric materials having a perovskite structure and bismuth-based oxide ferroelectric materials having a bismuth-layered structure have been known. Typical examples of the lead-based ferroelectric materials are PbZrxTi1-xO3 (PZT), PbyLa1-yZrxTi1-xO3 (PLZT) and the like. A typical example of the bismuth-based oxide ferroelectric materials is SrBi2Ta2O9 (BST).
The ferroelectric capacitor offers a higher charge retention capability as the polarization of the ferroelectric material is greater, and can retain the electric potential with less capacitance. Specifically, the FeRAM can be fabricated with high integration. Furthermore, as the polarization of the ferroelectric material is greater, the polarization directions can be differentiated more clearly even at a low reading-out voltage, thus enabling the ferroelectric memory to be driven at a low voltage.
It is effective to arrange orientations of ferroelectric crystals uniformly in order to increase a polarization amount of the ferroelectric material. For example, on pages 382 to 388 of “Journal of Applied Physics” 1991, vol. 70, No. 1, disclosed is a technology of obtaining a (111)-oriented ferroelectric thin film, in which metal thin films formed of metals such as platinum (Pt) and iridium (Ir) are deposited at 500° C. to obtain a (111)-oriented metal thin film, and a ferroelectric thin film such as PZT is deposited on this metal thin film at a room temperature, followed by heating of the deposited ferroelectric thin film to a range from 650° C. to 700° C. However, the maximum temperature permitted for a manufacturing process of the FeRAM is usually 620° C.
The ferroelectric material such as PZT having a tetragonal simple perovskite structure has a polarization axis along the c axis <001>. Accordingly, the polarization amount becomes maximum when the ferroelectric layer is approximately oriented along a (001) plane (hereinafter, referred to as (001)-oriented). When the ferroelectric layer is (111)-oriented, a component of the polarization produced in <001> direction is only about 1/1.73 in <111> direction that is a thickness direction of the ferroelectric layer. Although the polarization can be increased by aligning orientation, it is impossible to increase the polarization to the maximum.